1. Field of the Invention
The invention relates to a low temperature-fired porcelain with a low dielectric constant and a high quality coefficient Q, and to an electronic part using the porcelain.
2. Related Art Statement
A laminate type dielectric filter has been used as a high frequency circuit filter, such as a top filter, interstage filter for transmission, local filter and interstage filter for reception, in a high frequency circuit radio instrument such as a cellular phone. Examples of such laminate type dielectric filters are disclosed, for example, in JP 243810/1993A (laid-open publication).
The laminate type dielectric filter may be produced as follows. Ceramic powder for producing a dielectric material is shaped to provide a plurality of green sheets. A given conductive paste is printed on each green sheet to form a conductive paste layer with a predetermined electrode pattern on each green sheet. The obtained green sheets are then laminated to form a laminated body. The laminated body is then heated so that the green sheets and conductive paste layers are simultaneously sintered and densified to provide porcelain layers and metallic electrodes thereon.
The electrode is generally made of a metallic conductor with a low melting point, such as a silver-based conductor, copper-based conductor or nickel-based conductor. Such metallic conductor has a low melting point, for example, of not higher than 1100xc2x0 C. and sometimes as low as around 930xc2x0 C. It is therefore necessary to fire the laminate body at a firing temperature lower than the melting point of the metal constituting the electrode pattern.
It has been desired to provide a porcelain with a low optimum firing temperature (low temperature-fired porcelain) having a reduced dielectric constant ∈ r and improved quality coefficient Q. Such porcelain is indispensable for reducing the stray capacity, delay time and high frequency loss of an resonator and condenser housed. The assignee filed a Japanese Patent publication JP 211,969/2000A, and disclosed a low temperature fired porcelain with a optimum firing temperature not higher than 1000xc2x0 C., a dielectric constant of not higher than 10 and a quality coefficient of not higher than 2500.
In the porcelain disclosed in JP 211,969/2000A (EP 1022264A), specified amounts of zinc oxide and boron oxide that are added to a formulation for a low temperature fired porcelain of a BaOxe2x80x94SiO2xe2x80x94Al2O3 system. The resulting porcelain has a low optimum firing temperature, a dielectric constant of not higher than 10 and a quality coefficient of not higher than 2500. The inventors have performed intensive research for further improving the quality coefficient while preserving the low optimum firing temperature and low dielectric constant. The inventors, however, encountered the following problems. In the above-described porcelain, a glassy raw material containing boron oxide is added to the formulation for the porcelain to supply boron oxide. It is necessary to increase the amount of boron oxide in the glassy raw material to reduce the optimum firing temperature of the porcelain. As the content of boron oxide in the glassy raw material is increased, however, the quality coefficient tends to be reduced. It is thus difficult to further improve the quality coefficient of the porcelain over 2500. On the other hand, the quality coefficient may be improved by reducing the content of boron oxide. As the content of boron oxide is reduced, however, the thermal expansion and thermal contraction behaviors of the porcelain may be changed so that the porcelain is not applicable to various kinds of electronic parts, according to the inventor""s findings. Particularly when the metallic electrodes, for example made of silver, are formed between the porcelain layers as described above, cracks may occur in the porcelain layer along the interface between the porcelain layer and metallic electrode. Spaces may be also formed along the interface of the porcelain layer and metallic electrode. Such defects may result in off-specification products and thus reduce the production yield.
An object of the invention is to provide a low temperature fired porcelain having an optimum firing temperature not higher than 1000xc2x0 C., a reduced dielectric constant ∈ r, an improved quality coefficient and a low incidence of cracks.
The invention provides a low temperature-fired porcelain comprising a barium component in a calculated amount of 10 to 64 weight percent when calculated as BaO, a silicon component in a calculated amount of 20 to 80 weight percent when calculated as SiO2, an aluminum component in a calculated amount of 0.1 to 20 weight percent when calculated as Al2O3, a boron component in a calculated amount of 0.3 to 1.0 weight percent when calculated as B2O3, a zinc component in a calculated amount of 0.5 to 20 weight percent when calculated as ZnO, and a bismuth component in a calculated amount of not higher than 20 weight percent when calculated as Bi2O3.
The inventors have performed an extensive research and applied a glass with a low melting point containing B2O3 as one of raw materials, for providing a porcelain with a low optimum firing temperature. As the content of B2O3 in the glass is increased, however, the quality coefficient Q of the resulting porcelain tends to be reduced as described above. It was thus difficult to obtain a porcelain with a lower optimum firing temperature and a higher quality coefficient Q at the same time. When the content of B2O3 is reduced, the quality coefficient Q may be considerably improved. However, the thermal expansion and thermal contraction behaviors may be changed so that the porcelain may not be generally adapted for use in various kinds of electronic parts. In particular, the incidence of cracks may be increased in the porcelain along the interface between the porcelain layer and metallic electrode.
The inventors have applied, based on the findings described above, Bi2O3 with B2O3 in the formulation for producing a porcelain. According to the inventors"" discovery, the combination of Bi2O3 and B2O3 may be effective for improving the quality coefficient Q of a porcelain and for preventing the cracks in the porcelain at the same time. The invention is based on the discovery.